Semiconductor vibration detecting structure

Abstract

A semiconductor vibration detecting structure formed on a semiconductor substrate and a method of manufacturing the same in which the curvature of the cantilever of the vibration detecting structure in the direction gradually deviating from the surface of the semiconductor substrate can be determined by the thickness of the upper nitride layer for regulating the curvature of the cantilever, with the thicknesses of the other layers constituting the cantilever and the length thereof being constant. In the semiconductor vibration detecting structure according to the present invention, even if vibrations having relatively large vibration levels are applied to the detecting structure, the cantilever of the vibration detecting structure vibrates well but never hits the surface of the semiconductor substrate, thus enabling a wide rage of mechanical vibrations to be detected with a high sensitivity.

What is claimed is: 
     
       1. A semiconductor vibration detecting structure having a semiconductor substrate (1) of a first conductivity type, a plurality of semiconductor layers (3, 5, 7, 9) of the second conductivity type opposite to that of the substrate diffused in the substrate, and a cantilever (13) comprising a root portion fixed to the substrate (1) through an insulator film and a movable beam (19) projecting from the root portion, the structure comprising: a fixed capacitor portion (31) formed between the root portion of said cantilever and said semiconductor substrate through the insulator film; and   a variable capacitor portion (29) formed between the movable beam (19) of the cantilever and said semiconductor substrate, said cantilever gradually deflecting upward from the surface of said semiconductor layer towards the distal end of the cantilever.   
     
     
       2. The semiconductor vibration detecting structure as claimed in claim 1, wherein said cantilever comprising at least two layers having coefficients of thermal expansion different from each other. 
     
     
       3. The semiconductor vibration detecting structure as claimed in claim 2 wherein the semiconductor layer (17) of the cantilever is a boron-doped polysilicon layer and the other layer (15, 19) are nitride film layers coated on both surfaces of the polysilicon layer. 
     
     
       4. The semiconductor vibration detecting structure as claimed in claim 2 wherein the direction of the curvature of the free end portion of the cantilever is determined by the relationship among the thicknesses of the boron doped polysilicon layer and of the nitride film layers. 
     
     
       5. The semiconductor vibration detecting structure as claimed in claim 4 wherein the curvature (1) of the free end portion of the cantilever in the direction gradually deviating from the semiconductor substrate is determined by the thickness of the upper nitride film layer (19) for regulating the curvature of the cantilever, with the thicknesses of the boron-doped polysilicon layer (17), the lower nitride layer (15) and the length (L) of the cantilever being constant. 
     
     
       6. The semiconductor vibration detecting structure as claimed in claim 4, wherein the curvature (1) of the free end portion of the cantilever is formed in such that the cantilever (13) vibrates most, close to the upper surface of the semiconductor substrate coated with the oxide film but does not hit the upper surface of the oxide film, when vibration having a vibration frequency substantially equal to the resonance frequency of the cantilever is applied thereto. 
     
     
       7. The semiconductor vibration detecting structure as claimed in claim 5 wherein the curvature (1) of the free end portion of the cantilever (13) in the direction gradually deviating from the semiconductor layer increases according to a certain function, as the thickness of the upper nitride film layer for regulating the curvature is increased. 
     
     
       8. The semiconductor vibration detecting structure as claimed in any one of claim 2 wherein vibrations ranging from 1 G to 40 G can be detected, with the distance a between the lower surface of the cantilever at the root thereof and the upper surface of the oxide film being 1 μm, the distance b between the lower surface of the cantilever at the extreme end and the upper surface of the oxide film on the semiconductor substrate being 30 μm and the length L of the free end portion of the cantilever being 450 82 m. 
     
     
       9. The semiconductor vibration detecting structure as claimed in claim 8 wherein the resonance frequency of the cantilever is about 7 kHz where the maximum output voltage can be obtained from the output terminal of the MOS transistor portion of the detecting structure. 
     
     
       10. The semiconductor vibration detecting structure as claimed in claim 2 wherein at least one operational amplifier circuit is integrally formed on the same first time semiconductor substrate. 
     
     
       11. The semiconductor vibration detecting structure as claimed in claim 2 wherein said first type semiconductor substrate is made of N type semiconductor material and said second type semiconductor regions are made of P type semiconductor material. 
     
     
       12. The semiconductor vibration detecting structure as claim in claim 2, at least one of the layers of the cantilever is a deposited layer. 
     
     
       13. The semiconductor vibration detecting structure as claimed in claim 2, wherein, said cantilever is fabricated, under different temperature from that in use, as having a straight beam. 
     
     
       14. The semiconductor vibration detecting structure as claimed in claim 10 wherein said first type semiconductor substrate is made of N type semiconductor material and said second type semiconductor regions are made of P type semiconductor material.